The invention relates generally to the field of display technology, and in particular to a process to color calibrate the light output of a display.
This invention provides a process to color calibrate the light output response of a display. In today""s digital imaging world, many images are previewed and manipulated on softcopy displays. Imaging workstation displays are intended to simulate or match the look of hardcopy output, thus there is a need to calibrate softcopy displays to match hardcopy output. A process that has been used to calibrate displays used in high quality photographic imaging workstations is described in U.S. Pat. No. 5,371,537, xe2x80x9cMethod and Apparatus for Automatically Calibrating a CRT Display,xe2x80x9d A. E. Bohan et al., assigned to Eastman Kodak Company, issued Dec. 6, 1994. The patent by Bohan et al. teaches a method to calibrate a CRT display by mapping a CRT response curve to Aim Tone-Scale with the use of look-up tables for the individual channels. Such methods provide CRT calibration, but the calibration technique may limit the use of many digital code values when setting the display aim luminance and color to the application specification to get the display to the aim white point. Thus when the display brightness level continues to decrease over time, and further calibration is required, fewer and fewer code values are available, thereby restricting the total dynamic range per channel of the display. Display image quality can further suffer due to the quantizing effect of using fewer code values throughout the dynamic range. In the digital code value range one can only achieve the desired aim white level and color by decreasing the overall brightness of the display. In effect, the code values at the top of the range are sacrificed to achieve color balance.
There is a need therefore for an improved method of calibrating a multichannel soft copy display.
The present invention is directed to overcoming one or more of the problems set forth above. Briefly summarized, according to one aspect of the present invention, a method of calibrating a multichannel display device having an overall and individual channel adjustment for both gain and offset and an adjustment to provide a white point for the display, the white point including color temperature, chromaticity and luminance level, comprises the steps of: displaying a first target using a low level code value for each channel of the display; sensing the luminance level of the displayed first target; adjusting the gain of the display so that the sensed luminance level matches a first predetermined aim value representing a luminance level at least 3 decades lower than a maximum luminance level; displaying a second target using intermediate code values for each channel of the display device; sensing the luminance level and chromaticities of the displayed second target; adjusting the individual channel offsets so that the luminance level matches a second predetermined aim value representing an intermediate luminance level and the chromaticities match a first set of predetermined chromaticities that represent a desired white point; displaying a third target using maximum code values for each channel of the display; sensing the luminance level and chromaticities of the displayed third target; adjusting the individual channel gains so that the luminance level matches a third predetermined aim value the maximum luminance level and the chromaticities match the first set of predetermined chromaticities; and repeating the steps of displaying and adjusting until no further adjustmeent is required in the last step.
These and other aspects, objects, features and advantages of the present invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and by reference to the accompanying drawings.
The present invention has the advantage of setting the display luminance""s dynamic range to the dynamic range of a hardcopy media. For example, it is capable of achieving greater than 3 decades of luminance range. It has the further advantage that the display uses internal and external controls to set up the aim color white point for desired application. The display white chromatics are customized then to the white of the hardcopy media. Using the present invention, an aim calibration worksheet including input signal, with light output aims in terms of color chromaticities and luminance, with corresponding controls can be prepared so that all of the information regarding the color calibration can be displayed at once. Robust performance of Gray Scale tracking is achieved by trading off lowlights color performance.